You need player voting. Checking volume or polycount is meaningless as others have stated, to easy to bypass. And a bigger sword shouldnt by default be more epic/have better stats. Thats really bad design.
But why implement this at all? Seems you want to monetize like "second life" or something. But you do realize the game will look pretty ugly with no distinct art direction at all? Imaging WOW but all toons run around with player-made 3d-objects attached to them... In second life it works because there is no competition, just flaunting your expensive gear.
And you pay less for a more ugly sword/armour? So many players will buy just that (because who likes grinding for in-game money). Not a game I would like to play...
I mean it COULD work in a super-popular, super-populated gameworld if you strictly control the creation process with voting etc. I just dont think its realistic at all.